The present invention relates to a microwave landing system (MLS) and, more particularly, to an MLS in which a beam scanning antenna is provided with an amplifying function to serve as an active array and a traditional amplifier is omitted to enhance reliable operations.
An MLS has customarily been constituted by an elevation (EL) guidance equipment and an azimuth (AZ) guidance equipment with or without the addition of a back EL (BAZ) guidance equipment. The EL guidance equipment and AZ guidance equipment are essentially similar in construction to each other except for the arrangement of a beam scanning antenna. Specifically, while the beam scanning antenna of the AZ guidance equipment forms a vertically wide and horizontally narrow fan beam and moves it in a reciprocating motion in the azimuthal direction, the beam scanning antenna of the EL guidance equipment forms a horizontally wide and vertically narrow fan beam and reciprocatingly moves it in the elevational direction. The AZ guidance equipment and the EL guidance equipment are equipped with six auxiliary antennas and two auxiliary antennas, respectively. Both of the AZ guidance equipment and EL guidance equipment are made up of a controller for generating a system sync signal which governs the timings of the entire system and a timing signal which determines the operation timings of the guide itself, an exciter for generating a signal having a predetermined frequency to be used by the MLS and modulating a preamble of transmission timings by DPSK (Differential Phase Shift Keying) according to a predetermined sequence, a power amplifier for amplifying a high frequency signal generated by the exciter to a predetermined output, and an antenna switching unit for selectively feeding power to antennas. Each beam scanning antenna is generally implemented by a phased array. Specifically, each beam scanning antenna is made up of a power divider for dividing transmission power by a predetermined ratio, phase shifters for effecting phase shifts for beam scanning, and radiating elements.
The prior art MLS having a beam scanning antenna which is implemented as a phased array is provided with redundancy. However, a power amplifier, antenna switching unit and the like which are included in each of the AZ and EL guidance equipments and are the key to the reliability of operation are not provided with redundancy. Especially, when the system is constructed in a dual configuration, it is extremely difficult to provide the antenna switching unit with redundancy and therefore to enhance the reliability of operation.